This invention concerns mounting of manhole covers used to close off access to utility passages extending beneath city streets. For security purposes, it is desirable to limit access to such passages by locking the manhole covers onto their supporting seats.
However, manhole covers are sometimes subjected to very high pressures caused by explosions as when an accumulation of methane gas, etc. in the space below is ignited.
Manhole vault explosions usually blow the manhole covers out of their seats and into the air with great force. Since each manhole cover must be reinstalled as soon as possible after an explosion to cover up the hazardous open manhole, a significant maintenance cost is entailed.
Until a dislodged manhole cover can be replaced into its frame, the open manhole presents a serious hazard.
Pressure rises rapidly beneath a manhole cover in an explosion, and even a relatively small pressure rise will lift the manhole cover off its seat. For example, a momentary pressure rise of only one PSI beneath a 700 square inch manhole cover weighing 200 lbs. equates to a 500 lb. force available to dislodge the cover from its seat.
Although such explosive events are rare, when they do occur, manhole covers are often blown high into the air, can cause much damage, and even become deadly if a manhole cover strikes a passerby.
Typically, a metal manhole cover frame is cemented to the top of a manhole site chimney and set into the surrounding pavement.
A complicating factor is that the dislodging of the manhole cover acts to relieve gas pressure in the manhole during explosive events. Fixing manhole covers in place on their seats could cause damage to enclosing structures if there is no venting of the rapidly expanding gases. Pressure must somehow be relieved to avoid this potential structural damage.
It is an object of the present invention to provide a method which allows a controlled pressure relief while avoiding launching of the manhole cover out of its seat when an explosion occurs in the space beneath the cover.